Set counter of subfigures of figures

Question

This question builds off on the question here Subfigures in subfigures in latex enter link description here

However, what I would like to have is an additional level of subcaptions for the individual figures. Say, I would like (i) and (ii) for each of the figures in (a) and (b).

I tried modifying the code as follows:

\documentclass{article}
\usepackage{subcaption}
\usepackage{graphicx}

\begin{document}

\begin{figure}
\centering

\begin{subfigure}[b]{.4\textwidth}
\centering
\includegraphics[width=.4\textwidth]{example-image}
\subcaption{fig. (i)}
\includegraphics[width=.4\textwidth]{example-image}
\subcaption{fig. (ii)}
\caption{My compound subfigure (a)}
\end{subfigure}\quad
\begin{subfigure}[b]{.4\textwidth}
\centering
\includegraphics[width=.4\textwidth]{example-image}
\subcaption{fig. (i)}
    \quad
\includegraphics[width=.4\textwidth]{example-image}
\subcaption{fig. (ii)}
\caption{My compound subfigure (b)}
\end{subfigure}
\caption{A caption}
    
\end{figure}
    
\end{document}

But this was not correct and gave me the following:

Any suggestions on how I get the second layer of subcaptioning inside the subfigures?

Update: I would like to have (i) and (ii) in each subfigure side-by-side. Also, is it possible to automate the (i) and (ii), etc, this more for general reference for later generations?

Answer 1

Sorry, your question is not entirely clear (tome), so I guess, that you after the following:

For above images you need manually set subcaption counters:

\documentclass{article}
\usepackage{subcaption}
\usepackage{graphicx}

\begin{document}

\begin{figure}
\centering

\begin{subfigure}[b]{.4\textwidth}
\renewcommand\thesubfigure{\roman{subfigure}}
\includegraphics[width=\linewidth]{example-image-a}
\subcaption{fig. (i)}

\includegraphics[width=\linewidth]{example-image-b}
\subcaption{fig. (ii)}
\setcounter{subfigure}{0}
\renewcommand\thesubfigure{\alph{subfigure}}
\caption{My compound subfigure (a)}
\end{subfigure}
    \hfill
\begin{subfigure}[b]{.4\textwidth}
\setcounter{subfigure}{0}
\renewcommand\thesubfigure{\roman{subfigure}}
\includegraphics[width=\linewidth]{example-image-a}
\subcaption{fig. (i)}
    
\includegraphics[width=\linewidth]{example-image-b}
\subcaption{fig. (ii)}
\setcounter{subfigure}{1}
\renewcommand\thesubfigure{\alph{subfigure}}
\caption{My compound subfigure (b)}
\end{subfigure}

\caption{A caption}
\end{figure}

\end{document}

Answer 2

Something like this?

The subcaption package typesets the subfigure captions at \small, for a ca. 10% linear reduction in font size. In view of this factoid, I've chosen to insert \footnotesize directives at the top of each subfigure environment, for a further ca. 10% reduction in font size of the manually-numbered second-level captions.

I trust that having to enumerate the subsubcaptions manually won't be much of a chore, since the number of subsubfigures within each subfigure will be rather small -- 2 or max 3, right?

\documentclass{article}
\usepackage{subcaption}
\usepackage{graphicx}

\begin{document}
\begin{figure}[ht]

\begin{subfigure}[b]{.45\textwidth}
\centering\footnotesize

\includegraphics[width=\linewidth]{example-image}
(i) Some text

\bigskip
\includegraphics[width=\linewidth]{example-image}
(ii) Some more text

\caption{First compound subfigure}
\end{subfigure}%
\hfill% maximize the horizontal separation
\begin{subfigure}[b]{.45\textwidth}
\centering\footnotesize

\includegraphics[width=\linewidth]{example-image}
(i) Some text

\bigskip
\includegraphics[width=\linewidth]{example-image}
(ii) Some more text

\caption{Second compound subfigure}
\end{subfigure}

\caption{A figure with two compound subfigures}
\end{figure}
   
\end{document}